I know that I've previously mentioned what I think Cinder's Plan is on several occasions, but I've never fully written it out in detail. So instead of trawling through all my other theories to find the various sections of it, I decided to write it all down in one place.
Firstly, I should say that although I have an idea of what her plan is, I don't know her reasons for it yet. Hopefully, we'll find them out soon.
Anyway, so far we have seen certain parts of her plan: infiltrating Beacon, attending the Vytal festival and uploading a virus into the CCT's computer system, as well as lead an entire army of grimm into Vale. So, what are her reasons for this?
I personally believe that she intends to remove Ozpin from his position, by either having him demoted entirely, or killing him. This has been hinted at at various times, and would also make for good moment as the lights of Beacon go out. As well as this, Monty mentioned that Cinder was in opposition with Ozpin and knew that he was a very powerful Huntsman. This could mean that she was a student of his in the past (and would also explain her symbol being the Black Queen). However, this is more on her reasons for her actions which will probably be a separate post.
She has already made several moves to remove Ozpin. This can be seen in the fact that Ironwood already replaced his position as the military man for the festival. The reason she intends to have Ozpin replaced by Ironwood is because Ironwood is extremely easy to manipulate. He is hot headed, powerful and also has a romantic rivalry with Ozpin for Glynda. There isn't anyone who could be better for Cinder's aspirations to replace the current headmaster.
It has also been hinted that Cinder may attempt to take over the Altesian Knight programme using the Black Queen virus she uploaded. Penny would also likely be a part of the take over, and as she is also taking part in the Vytal festival, Cinder will have direct contact with her.
Another part of her plan is the collection of dust. Since the first episode, she has had Torchwick collecting huge amounts of dust for her. This could be for several reasons. She may desire to form some kind of alliance with the Schnee Company as they would probably appreciate having their dust returned to them, as well as other company's.
Or, she may intend to use it for its more basic abilities: making things explode. With the White Fang under her command, and with the ability to distract all of Vale, she could easily set up the dust in order to cause a detonation that would leave Vale vulnerable to the hordes of grimm outside, including all the Goliaths that aren't too far away. This would be catastrophic and would probably end up with Vale being utterly destroyed, especially if Ironwood's Altesian Knights have been reprogammed to not attack the grimm, and maybe even turn on the humans and faunus as well. Or of course, Ironwood may simply have already been corrupted and would have the Altesian Knights "fail to defeat the grimm".
With all this going on, it is likely that she will put that "list" of hers into action. If this list is a list of those she intends to either brainwash or corrupt, then she would do so now, creating an army of Vale's best Huntsmen and Huntresses to rule what's left of the hold. Alternatively, these may be people she intends to kill, and, well with all she has at her disposal, it wouldn't be difficult.
With this done, Vale would either be under her control or destroyed, and the events of the Vytal festival would cause distrust between all the remaining holds, thus plunging Remnant into chaos, and allowing her to take control, or let the grimm wipe them out.
And thus, she has basically destroyed every bit of hope left in the world, and it'll be up to RWBY, JNPR, SSSN and however many teachers are left to save the day.
By the way, Cinder may also be able to use the dust to control the individuals on her list.
Update: I also forgot that the arena is a massive structure filled with people that is vulnerably floating in the sky, and likely runs off of dust powered floating devices, that could easily be blown up with more dust. :\